Vehicle fuel efficiency is generally constrained by vehicle weight and vehicle operating conditions. For example, quick acceleration, heavy braking, excessive idling, high speed driving, cold weather, aerodynamic drag, electrical accessories, increased vehicle weight, and terrain may each degrade vehicle fuel efficiency. In addition, as vehicle power requirements increase, vehicle fuel efficiency generally decreases.
Moreover, vehicles are manufactured to operate within a specific fuel efficiency range for a given fuel type, and are generally powered by only one or two fuel types. Often, however, vehicle power or fuel efficiency may be mismatched for a desired operating condition. For example, a vehicle suited for hauling heavy loads is often unsuitable for fuel-efficient long-distance driving. Similarly, a vehicle powered by gasoline may be more or less fuel efficient than a vehicle powered by electricity under certain driving conditions.